Stents are frequently used in the medical field to open vessels affected by conditions such as stenosis, thrombosis, restenosis, vulnerable plaque, and formation of intimal flaps or torn arterial linings caused by percutaneous transluminal coronary angioplasty (PCTA). Stents are used not only as a mechanical intervention, but also as vehicles for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand a vessel wall. Stents may be capable of being compressed in diameter, so that they can be moved through small vessels with the use of a catheter or balloon-catheter, and then expanded to a larger diameter once they are at the target location. Examples of such stents include those described in U.S. Pat. No. 4,733,665 to Palmaz, U.S. Pat. No. 4,800,882 to Gianturco, U.S. Pat. No. 4,886,062 to Wiktor, U.S. Pat. No. 5,514,154 to Lau et al., and U.S. Pat. No. 5,569,295 to Lam.
A stent must have sufficient radial strength to withstand structural loads, such as radial compressive forces, imposed on the stent as it supports the walls of a vessel or other anatomical lumen. In addition, the stent must possess sufficient flexibility to allow for crimping, deployment, and cyclic loading from surrounding tissue. Also, a sufficiently low profile, that includes diameter and size of struts, is important. As the profile of a stent decreases, the easier is its delivery through an anatomical lumen, and the smaller the disruption in the flow of blood or other bodily fluid.
Stents made of bioresorbable polymers have been developed to allow for improved healing of the anatomical lumen. Examples of bioresorbable polymer stents include those described in U.S. Pat. No. 8,002,817 to Limon, U.S. Pat. No. 8,303,644 to Lord, and U.S. Pat. No. 8,388,673 to Yang. FIG. 1 shows an end segment of an exemplary bioabsorbable polymer stent 10 designed to be delivered through anatomical lumen using a catheter and subsequently expanded. Stent 10 has a cylindrical shape having central axis 12 and includes a pattern of interconnecting structural elements or struts 14. Axis 12 extends through the center of the cylindrical shape formed by struts 14. The stresses involved during compression and deployment are generally distributed throughout various struts 14 but are focused at the bending elements or strut junctions.
There are different types of struts 14. Struts 14 include a series of ring struts 16 that are connected to each other by bending elements 18. Ring struts 16 and bending elements 18 form sinusoidal rings 20 configured to be reduced and expanded in diameter. Rings 20 are arranged longitudinally and centered on axis 12. Struts 14 also include link struts 22 that connect rings 20 to each other. Rings 20 and link struts 22 collectively form a tubular scaffold of stent 10. Ring 20d is located the distal end of stent 10.
Bending elements 18 form a more acute angle when stent 10 is crimped to allow radial compression of stent 10 in preparation for delivery through an anatomical lumen. Bending elements 18 subsequently bend to form a larger angle when stent 10 is deployed to allow for radial expansion of stent 10 within the anatomical lumen. After deployment, stent 10 is subjected to static and cyclic compressive loads from surrounding tissue. Rings 20 are configured to maintain the expanded state of stent 10 after deployment.
Polymer stents are typically more flexible than metallic stents. While greater flexibility facilitates deliverability through tortuous anatomical lumen, flexibility of the polymer substrate material also requires individual struts of polymer stents to be thicker than struts of comparable metallic stents in order to meet requisite mechanical strength requirements. Thicker struts can result in a polymer stent having a larger stent scaffold profile, or outer diameter, during delivery through an anatomical lumen.
In some cases, calcification can be present on the interior surface of an anatomical lumen. Due to a larger stent scaffold profile and higher conformability of the scaffold against the walls of the anatomical lumen, sharp edges of polymer stent struts may catch on spicules of calcium present in the anatomical lumen. However, making stent struts thinner in an effort to reduce the chance of catching calcium on anatomical lumen walls, can impact the mechanical strength of the stent.
Accordingly, there is a continuing need for stent strut configurations and manufacturing methods that facilitate delivery of polymer stents.